3 Answers
3

When Leo Fender was designing the Strat in the early 50's, he was trying to get the sound of a pedal/console steel guitar in a conventional electric guitar. Keep in mind that Western Swing music was all the rage at the time, and especially so in California. This sound was bright and clear, so Leo angled the pickup to maximize the treble response.

This was also the reason for the Stratocaster's "tremolo" bridge (actually correctly termed a vibrato), to simulate the note-bending capabilities of a steel guitar.

It may be worth noting that for the exact same reasons mentioned in Alistair Maxwell's response for why it is angled as it is, there are some players who prefer to angle it the other way, although that is far less common. The closer a pickup is to the bridge the brighter it sounds, and Strat style single coil pickups are naturally bright anyway, so switching the angle will make the top strings a little warmer and allow the bass strings a bit more top end. By playing a right-handed Strat left-handed, Jimi Hendrix had this setup. As with all things musical, choose what works best for you :)

In short, to give a better treble response (the closer to the bridge, the more trebley the sound). It reduces bass that may ruin the sound, and gives a twangier brighter sound without sounding muddy. Fender were the first ones to introduce it, with the Broadcaster (later Telecaster) and it was such a great idea that it was put on the Stratocaster, and has been a standard ever since.